The invention relates generally to heading references for vehicles such as an aircraft and more specifically concerns an improved magnetometer for use as a heading reference for an aircraft.
Conventionally, when magnetometers are used as heading references in aircraft, they are mounted in some remote part of the aircraft as far away as possible from such magnetic anomalies as the engine and the electrical system. Although this is a laudable procedure, it requires at least two magnetometers at the remote location and some type of signal resolver on the pilot's control panel to allow course changes to be made. It is therefore an object of this invention to use a single magnetometer within reach of the pilot so that he can make course changes simply by rotating the magnetometer with respect to the aircraft.
The earth's magnetic field lies parallel to the earth's surface only at the magnetic equator, corresponding roughly to the geographic equator, and becomes vertical at the two magnetic poles. Within the United States, the direction of the field varies from about 60.degree. to 75.degree. from the horizontal. If a magnetic sensing element is used in the strapped-down mode its angular relation to the vertical component of the earth's field will change when an aircraft executes a banked turn and under certain conditions intolerable errors in the output of the sensor will occur. This effect for the conventional magnetic compass is called "northerly turning error" and can, at worst, result in an indicated turn in the opposite direction to that actually being made.
In conventional autopilot designs in which a magnetometer heading reference has been used, the difficulty of the northerly turning error has been avoided either by stabilizing the magnetometer element with a free gyro so that it remains in a horizontal plane despite any banking of the aircraft, or by combining the magnetometer with a free directional gyro. For the combination the free directional gyro is used as the heading reference of the autopilot, and the output of the magnetometer is used to monitor the drift of the gyro so that it may be corrected, either manually or automatically, during level flight. Both of these approaches are high-cost approaches and are not highly reliable. It is therefore the primary object of this invention to provide a simple, low-cost highly reliable means for compensating for the northerly turning error in a magnetometer used as a heading reference for a vehicle such as an aircraft.